The present invention relates to an apparatus for and a method of receiving diversity to be used at a base station or terminal station of a radio communication system, where a digital signal sequence is radio-transmitted in the Orthogonal Frequency Division Multiplexing (OFDM) transmission system, or at a receiving station of a broadcasting system where radio broadcasting by a digital signal sequence is carried out in the transmission system.
For terrestrial television broadcasting, the analog system has been used until now. However, shifting to the digital system is planned to start by the year of 2000.
Since one transmitting station of terrestrial television broadcasting has to cover a very large area, receiving side apparatus tends to be affected by multipath reflection wave propagation (multipath), which is characteristic of radio wave propagation and deteriorates the quality of images by causing a ghost on received images, giving rise to a serious problem.
The scale of this multipath is much larger than that of the multipath which is the object of radio communication system used for cellular phones, mobile phones, or the like. Consequently, even with adaptive automatic equalizers, which are regarded to be effective as a multipath countermeasure in a radio communication system of this type, this problem cannot be coped with any longer.
Accordingly, an orthogonal frequency division multiplexing (OFDM) transmission system, which is resistant enough, in principle, to a poor environment for multipath propagation and able to transmit information of high quality, is expected to be used as the digital transmission system for terrestrial television broadcasting.
In OFDM signals transmitted in this OFDM transmission system, guarded intervals, which are copied from a part of transmission waveform (symbol), have been provided to absorb multipath propagation with a interval length shorter than that of the guarded intervals so as to prevent the quality of received images from getting fatally deteriorated. This multipath-resistant transmission characteristic of the OFDM transmission system has attracted the notice of the broad-band radio communication system industry, such as multimedia communications, which will be developed in the public switched telephone network or private branch exchange, in addition to terrestrial television broadcasting, and the use of this system is under technical discussion.
Further, since the OFDM transmission system, utilizing this multipath-resistant transmission characteristics, enables the configuration of single frequency networks (SFN) which transmit the same content in the same frequency simultaneously, it is a very effective transmission system avoiding such waste in using frequencies as in conventional situations where transmitting frequencies have to be changed from area to area.
However, although the OFDM transmission system has excellent multipath-resistant transmission characteristics, it cannot secure perfect quality of reception without deterioration in reception caused by influences of intense frequency selective fading due to multipath propagation, Doppler shift which occurs on mobile reception, or influences of time fading.
In particular, in order to transmit images of quality high enough to meet the expectations of digital terrestrial television broadcasting or multimedia communications for the next generation (high definition images), much higher stability and quality are required in radio transmission technique than that for existing cellular phones, mobile phones, or the like whose main task is voice communications. A receiving apparatus which provides better reception characteristics is urgently needed.
Furthermore, for effective utilization of frequencies, techniques for transmitting high definition images having a large amount of information in a narrow radio band are required. The use of modulating systems which are highly efficient in a radio propagation environment with mobility taken into account, for example, a high efficiency modulating system such as the QAM modulating system.
However, a problem arises. Since a high efficiency modulating system of this type is low in noise-resistance or interference-resistance (weak in distortion), it cannot realize transmission of information of satisfactory quality at a receiving apparatus which receives radio waves while moving or at a remote place from a transmitting station or base station, because radio waves have to be received in a state of low signal-to-noise ratio or in a state where propagation of radio waves is distorted to deteriorate reception readily.
Particularly, for terrestrial television broadcasting, which covers an extremely large area, development of techniques to improve the reception quality is desired because deterioration in reception is a serious problem for it.
As a device to improve reception characteristic in a poor environment for multipath wave propagation or for mobile reception, diversity reception has conventionally been considered. In order to realize diversity reception, however, it is necessary to identify distortion in time fading in a poor environment for radio wave propagation (to estimate amount of change in the direction of time base), and to identify distortion in frequency-selective fading (to estimate amount of change in the direction of frequency base). In a combining diversity, in particular, efficient combine (combine at the maximum ratio) cannot be carried out unless distortion (in amplitude and phase) is accurately estimated for each receiver branch.
Conventionally, application of a combining diversity has not been positive due to difficulties in estimating this distortion.
Thus, when radio transmission of digital signal sequence is carried out in the orthogonal frequency division multiplexing transmission system, although its multipath-resistant transmission characteristic of the orthogonal frequency division multiplexing transmission system is utilized effectively, it is still necessary to devise a measure to improve deterioration of reception characteristic in a severe environment for multipath reflection wave propagation or to apply a measure to improve deterioration of reception characteristic in mobile reception in order to realize transmission of information in high quality and precision everywhere in a large area to cover. Particularly, in the future when transmission of image information is going to be a main task of multimedia communications or digital terrestrial broadcasting, it is indispensable to apply a high efficiency modulating system, such as QAM modulation, and to devise a measure to improve receiving characteristic in the orthogonal frequency division multiplexing transmission system. Development of a diversity receiving technique for OFDM signals is most necessary.
It is an object of the present invention to provide an apparatus for and a method of receiving diversity which can improve combining diversity performance by estimating distortion accurately for each receiver branch.
It is another object of the present invention to provide an apparatus for and a method of receiving diversity which can improve the reception of the orthogonal frequency division multiplexing signals, which has used high efficiency modulating system in the environment of multipath reflection wave propagation or of mobile reception.
It is another object of the present invention to provide an apparatus for and a method of receiving diversity wherein high quality transmission of information is realized even in a poor environment for radio propagation.
A method of receiving diversity according to the present invention comprises; a receiving step for receiving orthogonal frequency division multiplexing signals which have been radio-transmitted, a frequency transforming procedure for transforming frequencies of the orthogonal frequency division multiplexing signals to obtain a plurality of frequency spectrum signals, a weighting step for weighting on spectrum signals before of combining processing given by said frequency transforming step correspondingly to transmission channel frequency responses found from said transforming frequency spectrum signals, a combining step for combining the plurality of frequency spectrum signals to obtain combined frequency spectrum signals, and a demodulating procedure for demodulating digital signal sequence from the combined frequency spectrum signals.
An apparatus for receiving diversity according to the present invention comprises; receiving unit which receives orthogonal frequency division multiplexing signals which have been radio-transmitted, frequency transforming unit which obtains a plurality of frequency spectrum signals by transforming frequencies of said orthogonal frequency division multiplexing signals, weighting unit which weights on spectrum signals before of combining processing given by said frequency transforming unit correspondingly to transmission channel frequency response obtained from said transforming frequency spectrum signals, combining unit which obtains combined frequency spectrum signals by combining a plurality of said frequency spectrum signals, and demodulating unit which demodulates digital signal sequence from said combined frequency spectrum signals.
Other features and advantages of the present invention will become further apparent from the following description.